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Stream Solute Export and Biogeochemistry in the Luquillo Mountains, Puerto Rico

Stream Solute Export and Biogeochemistry in the Luquillo Mountains, Puerto Rico. Richard L. Brereton William H. McDowell University of New Hampshire. University of New Hampshire. El Yunque National Forest, PR. The only tropical rainforest in the U.S.

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Stream Solute Export and Biogeochemistry in the Luquillo Mountains, Puerto Rico

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  1. Stream Solute Export and Biogeochemistry in the Luquillo Mountains, Puerto Rico Richard L. Brereton William H. McDowell University of New Hampshire University of New Hampshire

  2. El Yunque National Forest, PR The only tropical rainforest in the U.S. 70% of runoff is withdrawn (Crook 2005) LTER since 1988 CZO since 2009

  3. Tropical MontaneWet Forest

  4. Two Very Different Watersheds Rio Mameyes Volcaniclastic bedrock 3.5 m/y of rain Lower avg. elevation Rio Icacos/Blanco Granodiorite bedrock 4.5 m/y of rain Higher avg. elevation

  5. Volcaniclastic: Rio Mameyes Bisley watersheds Q. Sonadora Granodiorite Rio Icacos Q. Guaba

  6. QuebradaSonadora - low flow

  7. Controls on stream chemistry • Major ions reflect lithology and sea salt • Dilution at high flows • Silica dilution is among highest measured (Godsey et al. 2009) • TSS increases with flow • Biogeochemical puzzle: Carbon and Nitrogen

  8. Shanley, McDowell, and Stallard 2011 “Boomerang” effect of DOC in the Icacos

  9. El Yunque, 1984

  10. 1989 – Hurricane Hugo Study Area Category 4 Hurricane: 226 km/hr sustained (Inches of rain)

  11. Bisley Experimental Watersheds Before Hugo After Hugo

  12. Georges, 1998 Category 3, 175 km/hr sustained

  13. Mameyes headwaters – 2005

  14. Weathering products and DOC:no response to hurricanes Icacos after Georges

  15. Nitrate response in two small watersheds after Georges (1998-2009) Volcaniclastic (Prieta) Quartz diorite (Guaba)

  16. Two larger basins,Sonadora (254 ha) and Icacos (326 ha) after Georges (1998-2009) Pre-Hurricanes 1983-1986 Rio Icacos Q. Sonadora

  17. What makes nitrate behave so differently from other solutes? Why the slower return to baseline in granodiorite watersheds?

  18. Biogeochemical controlson N flux to streams • Hydrologic flow path – groundwater? surface runoff? • Residence time – contact with soils • What is the matrix? • Redox conditions – climate • Riparian denitrification NO3 N2O N2

  19. Groundwater monitoring wells Transects across catena

  20. Bisley– volcaniclastic Riparian zone: Denitrification“hotspot” periodically anaerobic C source high retention time Modified from McDowell et al. 1992

  21. Icacos trib. – granodiorite watershed Key: NO3 NH4 Slope 550 35 Riparian Zone 450 30 20 370 20 560 5 560 Stream: 100 10 Flows into Icacos Modified from McDowell et al. 1992

  22. Icacos – slow recovery

  23. Conclusions • Luquillo stream chemistry reflects lithology overlain by vegetation, climate, disturbance history • Hurricanes cause dramatic shifts in forest biogeochemistry, reflected in stream nitrate peaks • Riparian zones hold the key to understanding long-term nitrogen dynamics in the Luquillo Mountains

  24. Acknowledgements • Funding from NSF-LTER, NSF Ecosystems, NSF-CZO, USFS IITF, UPR, UNH • Collaborators include F. Scatena, A. Lugo, D. Schaefer, C. Asbury, J. Merriam, J. Potter, and others • Field and laboratory assistance from M. Salgado, M.J. Sanchez, J. Bithorn, J. Merriam, J. Potter, J. Orlando, and others

  25. Questions? Key: NO3 NH4 Slope 550 35 Riparian Zone 450 30 20 370 20 560 5 560 Stream: 100 10 Flows into Icacos Modified from McDowell et al. 1992

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